The present invention relates to a ballast circuit for gaseous discharge lamps; and more particularly to a high frequency ballast, operating on conventional 60 hertz power and energizing the lamps at a higher frequency in the neighborhood of 20-25 KHz.
It is known and recognized that gaseous discharge lamps, such as fluorescent lamps and high-intensity discharge (HID) lamps convert electrical energy to light more efficiently at the higher frequency than at 60 or 120 Hz, such as 15-20 KHz.
Despite the economic incentive to energize gas discharge lamps with a higher frequency, and even though many solid state ballast circuits have been proposed in the literature and are otherwise known, there has been no solid state ballast circuit which has gained widespread commercial acceptance. There are perhaps many factors which have contributed to the lack of a commercial solid state, high-frequency ballast or inverter for gaseous discharge lamps, among which are the low initial cost of conventional ballasts due to the large volume and efficient production techniques which are employed, thereby at least partially offsetting any reduction in operating costs by reducing the initial outlay. Further, from a technical standpoint, when solid state ballasts were first developed, the power switches that were available for operating at the frequency and current levels required, were either too expensive or not reliable enough. Many of these problems have now been overcome due to advances in technology, and further, the increased cost of energy has emphasized the need for reducing operating costs over the long term.
Manufactures are, however, still faced with probleas. Among such problems is the need to provide protection for the various faults which may occur in a solid state ballast, such as excessive current or voltage in the thyristor power circuit, lamp over-voltage, loss of primary power, lamp failure, and so on.
Considering the various applications, such as fluorescent lamps, low and high pressure sodium, and metal halides, if a separate solid state ballast is required for each such application, and each ballast must include not only the numerous fault protection mechanisms, but also regulate lamp current during normal operation, provide for start up and re-strike during the period between cusps of primary power when oscillation may be extinguished, it can be seen that development costs would quickly get out of hand, and a manufacturer would not be able to employ the tested techniques of one system to the other, as is normally done during the course of development to enhance reliability of the commercial system.
Briefly, then, the present invention is directed to a solid state ballast circuit in which the primary logic control techniques for lamp current regulation and fault protection may be used in a wide range of applications including fluorescent lamps (two-, three- and four-lamp loads), low pressure sodium, high pressure sodium, metal halide, and mercury lamps. Further, the circuitry is readily adaptable to all commercially available voltages and frequencies. This is not to say that all such ballasts are interchangeable, but rather, only relatively minor changes are required for each application from a manufacturing standpoint, and the basic logic and fault detection techniques are common to all applications.
The present invention employs a thyristor/capacitor inverter bridge which energizes the lamp circuit with a high frequency oscillating voltage derived from full-wave rectified line voltage. The lamp circuit, in the case of a fluorescent ballast, may include two, three or four lamps.
Lamp current is regulated by a commutation timing and pulse shaping circuit which derives its timing from, and is synchronized with, zero crossings of the thyristor current. As used herein, "commutation" refers to the firing or conduction of the power thyristors in relation to the phase of the inverter (thyristor) current. The commutation timing and pulse shaping circuit has an inherent minimum delay from each zero crossing of the thyristor current to permit the thyristors to commutate during each cycle of high frequency oscillation. An output signal of a lamp current sensing amplifier delays firing or commutation of the thyristors beyond the previous zero crossing as a function of increasing lamp current. In other words, when lamp current is at a relatively low value, the current sensing amplifier will advance the commutation time of the power thyristor that is to be turned on to thereby add energy to the oscillating circuit feeding current to the lamps; and as lamp current builds up, the commutation timer increases the delay between a zero crossing and the commutation time, thereby regulating the current (and energy) coupled to the lamp power circuit. A binary circuit is clocked by the output of the commutation timing and pulse shaping circuit to steer the output pulse alternately to one of the power thyristors, thereby generating the oscillating mode of the high frequency inverter. The frequency of oscillation is determined primarily by the resonance of the power bridge capacitors and the inductance in the transformer delivering current to the lamp load.
When power is first turned on, an initialization circuit initiates a time delay to permit circuit stabilization, and then enables the high frequency inverter to begin energizing the lamp load with a gradual build-up of power. This initialization circuit is also used to re-commence operation after certain faults are detected, as explained further below.
A re-strike circuit generates pulses coupled to the commutation timer and pulse shaping circuit to re-start operation when the rectified line voltage falls below that which is necessary to sustain oscillation of the inverter circuit.
Fault detection circuits are included for detecting: (a) loss of primary power (one or more half cycles), (b) lamp over-voltage for a predetermined time, and (c) thyristor over-current. If lamp current or lamp voltage become excessive, the commutation timer is responsive to such condition for extending the commutation time of the power thyristors, thereby reducing energy coupled to the lamp load.
If one of the fault detection circuits detects a loss of primary power, excessive lamp voltage for a predetermined time or excessive thyristor current, the initialization circuit, mentioned above, is reset to inhibit inverter oscillation for the initial start up period, which should enable the fault condition to subside, and to then re-commence an initialization cycle. In addition, if the detected fault is excessive thyristor current, it may be indicative of "punch through", that is, a condition in which both thyristors are conducting at the same time. In this event, the power thyristors are commutated to the off state during the period of delay mentioned.
A fault count circuit determines whether a predetermined number of faults occurs successively within a preset time; and if so, the inverter is shut down and must be reset manually, as by turning off the main power and turning it back on.
The present invention thus includes logic and control circuitry as well as power circuitry which may be adapted, with only minor variation to the various applications which a commercial solid state ballast must accommodate. Further, as will be more fully explained below, the logic and control section, although employing some analog circuits, uses primarily digital circuit techniques with their enhanced reliability and accuracy. All of the circuits used in the logic and control portion of the system are amenable to commercialization using modern integrated circuit techniques so that they can be incorporated on a single "chip". By using the chip in many hours of testing and actual commercial applications, the experience gained can be used to enhance reliability and efficiency for not only fluorescent lamp applications but for HID applications as well.
Additional advantages, some of which are inherent at least to some extent in earlier suggested solid state ballasts include: the ability to dim the lamp with simple circuit modification, either manually or automatically in response to ambient conditions to reduce energy consumption; cooler operation and reduced power loss due to more efficient conversion of electric power to light, resulting in reduced load on air conditioning systems; reduced size and weight of the ballasts resulting in lower shipping and installation costs; and more quiet operation.
Other features and advantages of the present invention will be apparent to persons skilled in the art from the following detailed description of a preferred embodiment accompanied by the attached drawing wherein identical reference numerals will refer to like parts in the various views.